1. Field of the Invention
This invention relates to using imaging methods for predicting fracture risk and/or location based on radiographs.
2. Description of the Related Art
Osteoporosis is among the most common conditions to affect the musculoskeletal system, as well as a frequent cause of locomotor pain and disability. Osteoporosis can occur in both human and animal subjects (e.g., horses). Osteoporosis (OP) occurs in a substantial portion of the human population over the age of fifty. The National Osteoporosis Foundation estimates that as many as 44 million Americans are affected by osteoporosis and low bone mass. In 1997, the estimated cost for osteoporosis related fractures was $13 billion. That figure increased to $17 billion in 2002 and is projected to increase to $210-240 billion by 2040. Currently it is expected that one in two women over the age of 50 will suffer an osteoporosis-related fracture.
Imaging techniques are important diagnostic tools, particularly for bone related conditions such as osteoporosis. Currently available techniques for the noninvasive assessment of the skeleton for the diagnosis of osteoporosis or the evaluation of an increased risk of fracture include dual x-ray absorptiometry (DXA) (Eastell et al. (1998) New Engl J. Med 338:736-746); quantitative computed tomography (QCT) (Cann (1988) Radiology 166:509-522); peripheral DXA (pDXA) (Patel et al. (1999) J Clin Densitom 2:397-401); peripheral QCT (pQCT) (Gluer et al. (1997) Semin Nucl Med 27:229-247); x-ray image absorptiometry (RA) (Gluer et al. (1997) Semin Nucl Med 27:229-247; and U.S. Pat. No. 6,246,745); and quantitative ultrasound (QUS) (Njeh et al. “Quantitative Ultrasound: Assessment of Osteoporosis and Bone Status”, 1999, Martin-Dunitz, London England; WO 9945845; WO 99/08597; and U.S. Pat. No. 6,077,224 which is incorporated herein by reference in its entirety).
DXA of the spine and hip has established itself as the most widely used method of measuring bone mineral density (BMD). Tothill, P. and D. W. Pye, (1992) Br J Radiol 65:807-813. The fundamental principle behind DXA is the measurement of the transmission through the body of x-rays of 2 different photon energy levels. Because of the dependence of the attenuation coefficient on the atomic number and photon energy, measurement of the transmission factors at 2 energy levels enables the area densities (i.e., the mass per unit projected area) of 2 different types of tissue to be inferred. In DXA scans, these are taken to be bone mineral (hydroxyapatite) and soft tissue, respectively. However, it is widely recognized that the accuracy of DXA scans is limited by the variable composition of soft tissue. Because of its higher hydrogen content, the attenuation coefficient of fat is different from that of lean tissue. Differences in the soft tissue composition in the path of the x-ray beam through bone compared with the adjacent soft tissue reference area cause errors in the BMD measurements, according to the results of several studies. Tothill, P. and D. W. Pye, (1992) Br J Radiol, 65:807-813; Svendsen, 0. L., et al., (1995) J Bone Min Res 10:868-873. Moreover, DXA systems are large and expensive, ranging in price between $75,000 and $150,000.
Quantitative computed tomography (QCT) is usually applied to measure the trabecular bone in the vertebral bodies. Cann (1988) Radiology 166:509-522. QCT studies are generally performed using a single kV setting (single-energy QCT), when the principal source of error is the variable composition of the bone marrow. However, a dual-kV scan (dual-energy QCT) is also possible. This reduces the accuracy errors but at the price of poorer precision and higher radiation dose. Like DXA, however, QCT are very expensive and the use of such equipment is currently limited to few research centers.
Quantitative ultrasound (QUS) is a technique for measuring the peripheral skeleton. Njeh et al. (1997) Osteoporosis Int 7:7-22; and Njeh et al., Quantitative Ultrasound: Assessment of Osteoporosis and Bone Status, 1999, Martin Dunitz, London, England. There is a wide variety of equipment available, with most devices using the heel as the measurement site. A sonographic pulse passing through bone is strongly attenuated as the signal is scattered and absorbed by trabeculae. Attenuation increases linearly with frequency, and the slope of the relationship is referred to as broadband ultrasonic attenuation (BUA; units: dB/MHz). BUA is reduced in patients with osteoporosis because there are fewer trabeculae in the calcaneus to attenuate the signal. In addition to BUA, most QUS systems also measure the speed of sound (SOS) in the heel by dividing the distance between the sonographic transducers by the propagation time (units: m/s). SOS values are reduced in patients with osteoporosis because with the loss of mineralized bone, the elastic modulus of the bone is decreased. There remain, however, several limitations to QUS measurements. The success of QUS in predicting fracture risk in younger patients remains uncertain. Another difficulty with QUS measurements is that they are not readily encompassed within the WHO definitions of osteoporosis and osteopenia. Moreover, no intervention thresholds have been developed. Thus, measurements cannot be used for therapeutic decision-making.
There are also several technical limitations to QUS. Many devices use a foot support that positions the patient's heel between fixed transducers. Thus, the measurement site is not readily adapted to different sizes and shapes of the calcaneus, and the exact anatomic site of the measurement varies from patient to patient. It is generally agreed that the relatively poor precision of QUS measurements makes most devices unsuitable for monitoring patients' response to treatment. Gluer (1997) J Bone Min Res 12:1280-1288.
Radiographic absorptiometry (RA) is a technique that was developed many years ago for assessing bone density in the hand, but the technique has recently attracted renewed interest. Gluer et al. (1997) Semin Nucl Med 27:229-247. With this technique, BMD is measured in the phalanges.
Furthermore, current methods and devices do not generally take into account bone structure analyses. See, e.g., Ruttimann et al. (1992) Oral Surg Oral Med Oral Pathol 74:98-110; Southard & Southard (1992) Oral Surg Oral Med Oral Pathol 73:751-9; White & Rudolph, (1999) Oral Surg Oral Med Oral Pathol Oral Radiol Endod 88:628-35.
BMD does not accurately predict the presence of osteoporotic fracture. See, e.g., Riggs et al. (1982) J Clin Invest 70:716-723; Krolner, B. and S. P. Nielsen (1982) Clin Sci. 62:329-336; Ott et al. (1987) J Bone Miner Res, 2:201-210; and Pacifici et al. (1987) J Clin Endocrinol Metab, 64:209-214. While BMD is correlated with long-term fracture risk in population based studies (Kains (1994) Osteoporosis Int 4:368-381), it cannot take into account factors that vary from patient to patient and that are major determinants of individual failure load and resultant fracture (Hayes, W. C. and M. L. Bouxsein, Biomechanics of cortical and trabecular bone: Implications for assessment of fracture risk, in Basic Orthopaedic Biomechanics, V. C. Mow and W. C. Hayes, Editors, 1997, Lippincott-Raven Publishers: Philadelphia, p. 69-111; Kroonenberg et al. (1995) J Biomech Eng. 117(3):309-318; Kroonenberg et al. (1996) Biomechanics 29(6):807-811; and Robinovitch et al. (1991) J Biomech Eng. 113:366-374). These factors include bone architecture and structure, bone morphology, and biomechanical loading and impact load. Indeed, patients receiving osteoclast inhibiting, anti-resorptive drugs show remarkable reductions in incident osteoporotic fractures by 60-65% but only small changes in BMD on the order of 4.0-4.5% (Reginster et al. (2000) Osteoporosis Int., 11(1):83-91; and Harris et al. (1999) JAMA 14:1344-1352), strongly indicating a significant discrepancy between clinical outcomes and BMD measurements of bone health.
Thus, there remains a need for compositions and methods for predicting fracture risk.